This application is based on Japanese Patent Application No. 8-340288 filed Dec. 20, 1996, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to techniques for effecting an interference fit of two parts.
2. Discussion of the Related Art
One example of known techniques for effecting an interference fit of two parts is disclosed in JP-U-63-144136 (Japanese Utility Model Application as published on Sep. 22, 1988), wherein a first part 200 is held stationary, and a second part 202 is forced by a rigid pressure head 204, into engagement with the first part 200, as shown in FIG. 12, to thereby achieve an interference fit of the first and second parts 200, 202.
In the known technique shown in FIG. 12, a mechanical force F is applied to the second part 202 by abutting contact of the pressure head 204 with one of the opposite ends of the second part 202 which is remote from the first part 200. According to this arrangement, the compressive stress acting on the second part 202 in its axial direction as a result of the abutting contact of the second part 202 with the first part 200 is substantially evenly distributed over the entire length of the second part 202, as indicated in the graph given in the lower part of FIG. 12. Thus, this conventional technique suffers from a problem that the axial portion of the second part 202 other than its end portion which engages the first part 200 for the interference fit is subject to the axial compressive stress.
In an effort to solve the above problem, the assignee of the present application developed a technique wherein at least one of the two parts is accelerated toward the other or each other in a direction that permits the interference fit of the two parts, to apply a kinetic energy to the above-indicated at least one part, for thereby achieving the interference fit of the two parts, as disclosed in co-pending Application, Ser. No. 08/670,743 filed Jun. 21, 1996. According to this technique, the axial compressive stress acting on the second part 202 as a result of the abutting contact with the first part 200 continuously decreases in the axial direction from the axial end of the second part 202 abutting on the first part 200 toward the other axial end of the second part 202, at which the stress is zero, as indicated in FIG. 13.
The present applicants had the following finding as a result of continued research of the technique as disclosed in the above-identified co-pending application. That is, it was found that the thrust force applied to the second part immediately after the initiation of the acceleration of the second part tends to vary or fluctuate unless the thrust force to be applied upon the initiation of the acceleration is positively controlled. The variation of the initial thrust force applied to the second part causes a variation in the velocity of the second part upon abutting contact of the second part with the first part, leading to deteriorated stability in the accuracy of the interference fit of the two parts.
It is therefore a first object of the present invention to provide a method of effecting an interference fit of a first and a second part by accelerating the second part into abutting contact with the first part, which method permits improved stability of the thrust force to be applied to the second part upon initiation of acceleration of the second part.
This first object may be achieved according to a first aspect of the present invention, which provides a method of effecting an interference fit of a first part and a second part, comprising: a thrust force applying step of applying a thrust force to the second part in a direction toward the first part to thereby accelerate the second part for applying a kinetic energy to the second part to cause abutting contact of the second part with the first part for achieving the interference fit of the first and second parts; and an acceleration initiation inhibiting and permitting step of inhibiting initiation of acceleration of the second part by the thrust force until the thrust force has been increased to a predetermined threshold, and permitting the initiation of the acceleration when the thrust force has been increased to the predetermined threshold.
In the present method of effecting the interference fit of the two parts, the initiation of the acceleration of the second part is positively inhibited until the thrust force applied to the second part has been increased to the predetermined threshold or lower limit, so that the actual thrust force applied to the second part upon initiation of the acceleration is stabilized at a sufficient value, whereby the velocity of movement of the second part immediately before the abutting contact of the second part with the first part is accordingly stabilized to assure improved stability in the accuracy of the interference fit of the first and second parts.
The first and second parts may both have mutually fitting portions that are rigid. Alternatively, one and the other of these mutually fitting portions of the two parts may be rigid and elastic, respectively.
Where one of the two parts has an end portion in the form of a rod or shaft while the other part has a hole in which the end portion of the above-indicated one part is fitted with an interference, the above-indicated one part having the rod-shaped end portion or the other part having the hole may be accelerated. In other words, the above-indicated second part may be a part having a rod-shaped end portion or a hole. The second part may be accelerated by direct application of a force based on a pressure of a gas or liquid, or by indirect application of a force such as a magnetic force, a gravitational force or a centrifugal force. In one form of the method wherein the acceleration of the second part is effected by a pressurized gas, the pressurized gas delivered from a high-pressure source is introduced through a suitable gas passage into a rear space formed at the rear of the second part which is accommodated in a guide passage such that the second part may be guided toward the first part. The above-indicated rear space is defined by at least the rear end face of the second part remote from the first part and a rear end portion of the guide passage remote from the first part. It is desirable to terminate the acceleration of the second part to permit a constant-velocity or uniform motion of the second part immediately before the abutting contact of the second part with the first part, in order to assure that the axial compressive stress at the rear end of the second part remote from the first part is substantially zero upon abutting contact of the second part with the first part.
In one preferred form of the method of the invention, the acceleration initiation inhibiting and permitting step comprises inhibiting the initiation of the acceleration of the second part by engagement of a stop member with the second part.
In another preferred form of the present method, the acceleration initiation inhibiting and permitting step comprises inhibiting the initiation of the acceleration of the second part by clamping the second part.
In a further preferred form of the present method, the acceleration initiation inhibiting and permitting step comprises inhibiting the initiation of the acceleration of the second part by forcing the second part onto a stationary member to thereby generate a force of friction between the second part and the stationary member.
A second object of the present invention is to provide an apparatus for effecting an interference fit of a first and a second part by accelerating the second part into abutting contact with the first part, which apparatus permits improved stability of the thrust force to be applied to the second part upon initiation of acceleration of the second part.
The above second object may be achieved according to a second aspect of the invention, which provides an apparatus for effecting an interference fit of a first part and a second part, comprising: a thrust force applying device for applying a thrust force to the second part in a direction toward the first part to thereby accelerate the second part for applying a kinetic energy to the second part to cause abutting contact of the second part with the first part for achieving the interference fit of the first and second parts; and an acceleration initiation inhibiting and permitting device for inhibiting initiation of acceleration of the second part by the thrust force until the thrust force has been increased to a predetermined threshold, and permitting the initiation of the acceleration when the thrust force has been increased to the predetermined threshold.
In the present apparatus for effecting the interference fit, the initiation of the acceleration of the second part is positively inhibited by the acceleration initiation inhibiting and permitting device until the thrust force applied to the second part has been increased to the predetermined threshold or lower limit, so that the actual thrust force applied to the second part upon initiation of the acceleration is stabilized, whereby the velocity of movement of the second part immediately before the abutting contact of the second part with the first part is accordingly stabilized to assure improved stability in the accuracy of the interference fit of the first and second parts.
In one preferred form of the apparatus of the present invention, the acceleration initiation inhibiting and permitting device comprises a movable member movable to an inhibiting position in which the movable member engages the second part to inhibit the initiation of the acceleration of the second part, and a permitting position in which the movable member is spaced apart from the second part to permit the initiation of the acceleration of the second part.
In one advantageous arrangement of the above preferred form of the apparatus, the movable member includes a stop member movable to a stop position as the inhibiting position in which the stop member engages a surface of the second part which faces the first part, to thereby inhibit the initiation of the acceleration of the second part, and a non-stop position as the permitting position in which the stop member is spaced apart from the surface of the second part, to thereby permit the initiation of the initiation of the second part, and the acceleration initiation inhibiting and permitting device further comprises an actuator for moving the stop member to the stop position and the non-stop position.
In another advantageous arrangement of the above form of the apparatus, the movable member comprises a clamping member operable to a clamp position in which the clamping member is held in pressing contact with a surface of the second part for holding the second part in a direction intersecting the surface, and an unclamp position in which the clamping member is spaced apart from the surface, and the acceleration initiation inhibiting and permitting device further comprises an actuator for operating the clamping member to the clamp and unclamp positions. The clamping member may be a collet of a collet chuck, for example.
In a further advantageous arrangement of the above form of the apparatus the apparatus further comprises a stationary housing having a guide passage for slidably guiding the second part, and the movable member comprises a presser member movable to an advanced position in which the presser member forces the second part at a surface thereof facing an inner surface of the guide passage, onto the inner surface, to thereby inhibit the initiation of the acceleration of the second part, and a retracted position in which the presser member is spaced apart from the surface of the second part, to thereby permit the initiation of the acceleration of the second part. In this case, the acceleration initiation inhibiting and permitting device further comprises an actuator for moving the presser member to the advanced and retracted positions.
A third object of this invention is to provide an apparatus for effecting effecting an interference fit of a first and a second part by accelerating the second part into abutting contact with the first part, which apparatus is capable of controlling in a feedback manner the thrust force to be applied to the second part upon initiation of acceleration of the second part, to thereby improve the stability of the thrust force upon the initiation of the acceleration.
The above object may be achieved according to a third aspect of the present invention, which provides an apparatus for effecting an interference fit of a first part and a second part, comprising: a thrust force applying device for applying a thrust force to the second part in a direction toward the first part to thereby accelerate the second part for applying a kinetic energy to the second part to cause abutting contact of the second part with the first part for achieving the interference fit of the first and second parts; and an acceleration initiation inhibiting and permitting device for inhibiting initiation of acceleration of the second part by the thrust force until the thrust force has been increased to a predetermined threshold, and permitting the initiation of the acceleration when the thrust force has been increased to the predetermined threshold, wherein the acceleration initiation inhibiting and permitting device comprises (a) a movable member movable to an inhibiting position in which the movable member engages the second part to inhibit the initiation of the acceleration of the second part, and a permitting position in which the movable member is spaced apart from the second part to permit the initiation of the acceleration of the second part, and (b) a moving device for moving the movable member to the inhibiting position until the thrust force has been increased to the predetermined threshold, and to the permitting position when the thrust force has been increased to the predetermined threshold.
In the present apparatus, the thrust force actually acting on the second part is fed-back to control the thrust force applied to the second part upon initiation of the acceleration, so that the accuracy of control of the initial thrust force applied to the second part is significantly improved.
The moving device may be an electrically operated actuator for moving the movable member on the basis of an output signal of a sensor, which signal relates to or represents the thrust force applied to the second part. Alternatively, the moving device may be a mechanically operated actuator for moving the movable member on the basis of a force which relates to or represents the thrust force applied to the second part. Thus, the moving member may be either electrically or mechanically associated with the movable member.
In one preferred form of the apparatus according to the third aspect of the invention, the moving device comprises: a sensor generating an output signal indicative of a quantity relating to the thrust force applied to the second part, an actuator for moving the movable member to the inhibiting and permitting positions; and a controller connected to the sensor and the actuator, for controlling the actuator on the basis of the output signal of the sensor, to hold the movable member in the inhibiting position until the thrust force has been increased to the predetermined threshold, and move the movable member to the permitting position when the thrust force has been increased to the predetermined threshold.
In one advantageous arrangement of the above preferred form of the apparatus, the thrust force applying device includes a pressurizing device for raising a pressure acting on the second part in the direction toward the first part, to a level higher than an atmospheric pressure, for thereby applying the thrust force to the second part, and the sensor comprises a pressure sensor whose output signal the indicates the pressure acting on the second part, as the quantity, the controller controlling the actuator on the basis of the output signal of the pressure sensor.
In the above advantageous arrangement, the pressurizing device may comprise: a high-pressure source for delivering a pressurized gas; a structure for defining a guide passage for substantially gas-tightly and slidably guiding the second part toward the first part; and means for defining a gas passage connecting the high-pressure source and one of opposite end portions of the guide passage which is remote from the first part. In this instance, it is preferable to provide a sealing member held in gas-tight contact with an outer surface of the second part and an inner surface of the guide passage, in order to avoid leakage of the pressurized gas through a gap between the outer surface of the second part and the inner surface of the guide passage when the pressurized gas is introduced from the high-pressure source into the end portion of the guide passage remote from the first part.
In another preferred form of the apparatus according to the third aspect of this invention, the thrust force applying device comprises a pressurizing device for raising a pressure acting on the second part in the direction toward the first part, to a level higher than an atmospheric pressure, for thereby applying the thrust force to the second part, and the moving device comprises an actuator which receives as a pilot pressure the pressure acting on the second part and which is operated to hold the movable member in the inhibiting position until the pilot pressure has been raised to a predetermined level and to move the movable member to the permitting position when the pilot pressure has been raised to the predetermined level.